In small electronic appliances such as note-type personal computers, smartphones, mobile phones, etc., which have been provided with increasingly higher performance and more functions, electronic devices such as microprocessors, imaging chips, memories, etc. should be mounted densely. Accordingly, to prevent malfunction due to heat generated by them, the dissipation of heat generated from such electronic devices has become increasingly important.
As a heat-dissipating sheet for electronic devices, JP 2006-306068 A discloses a heat-conductive sheet comprising at least a graphite film and an adhesive resin composition, which is a reaction-curable vinyl polymer. The graphite film is (a) expanded graphite formed by an expanding method, or (b) obtained by heat-treating a polyimide film, etc., at a temperature of 2400° C. or higher. The expanded graphite film is obtained by immersing graphite in acid such as sulfuric acid, etc. to form a graphite interlayer compound, heat-treating the graphite interlayer compound to foam it, thereby separating graphite layers, washing the resultant graphite powder to remove acid, and rolling the resultant thin-film graphite powder. However, the expanded graphite film has insufficient strength. Also, the graphite film obtained by the heat treatment of a polyimide film, etc. is disadvantageously expensive despite high heat dissipation.
JP 2012-211259 A discloses a heat-conductive sheet comprising graphite pieces, which comprise pluralities of first graphite pieces obtained by thinly cutting a thermally decomposed graphite sheet, and second graphite pieces smaller than the widths of the first graphite pieces, at least the first graphite pieces connecting both surfaces of the heat-conductive sheet. This heat-conductive sheet is obtained, for example, by blending the first and second graphite pieces with a mixture of an acrylic polymer and a solvent, and extruding the resultant blend. However, the extruded heat-conductive sheet does not have sufficient heat dissipation, because of a high volume fraction of the resin.
JP 2006-86271 A discloses a heat-dissipating sheet as thick as 50-150 μm comprising graphite bonded by an organic binder having a glass transition temperature of −50° C. to +50° C., such as an amorphous copolyester, a mass ratio of graphite/binder resin being 66.7/33.3 to 95/5. This heat-dissipating sheet is produced by applying a slurry of graphite and an organic binder in an organic solvent to a parting-agent-coated film on the side of a parting layer, drying the slurry by hot air to remove the organic solvent, and then pressing it, for example, at 30 kg/cm2. JP 2006-86271 A describes that the pressing of a graphite/organic binder sheet improves its thermal conductivity. However, because this heat-dissipating sheet contains an organic binder, it does not sufficiently exhibit high thermal conductivity inherent in graphite.
JP 11-1621 A discloses a high-thermal-conductivity, solid composite material for a heat dissipater comprising highly oriented graphite flakes and a binder polymer polymerized under pressure. This solid composite material is produced by mixing graphite flakes with a thermosetting monomer such as an epoxy resin to prepare a composition comprising at least 40% by volume of graphite, and polymerizing the monomer while compressing the composition under sufficient pressure to align graphite substantially in parallel. However, because this solid composite material comprises an epoxy resin, it does not have sufficiently high thermal conductivity.
JP 2012-136575 A discloses a conductive, heat-dissipating sheet comprising organic particles made of polyamides, acrylic resins, etc. and having an average particle size of about 0.1-100 μm, conductive inorganic fillers having an average particle size of about 10 nm to about 10 μm, and a cured resin such as an epoxy resin, etc., organic particles/inorganic fillers being 1000/1 to 10/1, and the percentage of inorganic fillers being 5-30% by weight based on the total amount. JP 2012-136575 A illustrates graphite, coke, carbon black, etc. as inorganic fillers, though only carbon black is used in Examples. In addition, this conductive heat-dissipating sheet does not have sufficient heat dissipation, because it contains the cured resin.
As described above, conventional heat-dissipating sheets containing graphite or carbon black do not have sufficient heat dissipation because they also contain binder resins. Increase in the percentage of graphite or carbon black results in lower sheet strength despite improved thermal conductivity, particularly causing the problem of easy detachment of graphite or carbon black from the heat-dissipating sheet. Accordingly, inexpensive heat-dissipating sheets having uniform, high heat dissipation as well as mechanical properties necessary for handling are desired.